Conventionally, in inverters and converters for motor driving, there is adopted a configuration in which two switching elements are connected in series, an input voltage is applied to both ends thereof, and an output is obtained from a midpoint. For example, if, in the case of a three phase inverter, three sets including six switching elements (three sets of switching element groups each consisting of two switching elements) are provided to an input voltage, the six switching elements are turned ON and OFF, and a three phase current is outputted from each midpoint of the sets of the switching element groups.
Furthermore, in such a circuit, pulse width modulation (PWM) control is performed to control the output current. In the PWM control, a normal triangular wave is compared with a command voltage and an on period of the switching elements is determined. Then, it is possible to control the outputs to be a predetermined one by changing the command voltage.
However, in such a power conversion circuit using switching elements, the power supply will be short circuited if upper and lower switching elements are turned ON at the same time, and, therefore, care must be taken to avoid such a situation from occurring. For this reason, a dead time in which both the upper and lower switching elements are turned OFF in switching is provided.
In configurations wherein a dead time is provided, when a current is allowed to flow from a midpoint of two switching elements connected in series between an upper power supply line and a lower power supply line, the current is directed through a diode connected in reversely parallel to the lower power supply line during the dead time period; as such, errors in which the output voltage decreases during the dead time period are generated in the command voltage. Furthermore, during the dead time period, at the time when the current is flowing through the midpoint, the current is directed through a diode connected in reversely parallel to the upper power supply line, such that errors in which the output voltage increases for the dead time period are generated in the command voltage. Because the current which flowing through the midpoint is near zero, this current will not effect the output voltage to the command voltage. A technique which uses this to compensate for the dead time is proposed in the prior art.
In Patent Document 1 (Japanese Patent Application Laid-Open No. 10-285937), at the end of a dead time, a voltage at an output terminal is measured, a direction of a load current connected thereto is determined by whether or not the measured voltage exceeds a threshold, and compensation for the dead time is determined in consideration of the determined result.
Furthermore, in Patent Document 2 (Japanese Patent Application Laid-Open No. 2004-248480), a current flowing through a load is detected by a current sensor, a direction of the current is determined, and dead time is compensated for in consideration of the determined result.
In the art described in Patent Document 1, when a voltage between upper and lower lines is changed, a voltage value of a voltage waveform at the output end is also changed. Therefore, there are situations wherein the direction of the load current cannot be correctly detected by a method in which the voltage at the output end is compared with the threshold having a constant value.
Furthermore, in the art described Patent Document 2, when there is an offset error in the current sensor or a current ripple is large, the current direction cannot be correctly determined.